p j m@see _my _sig _for_address.com wrote:

Oh, yes, absolutely.

The TBP is the point where the output of the heat pump is
sufficient to maintain target temps inside ( regardless of cost ).

The ECB is the point where the cost of getting a BTU out of
the HP exceeds the cost of getting it from some other source ( gas /
oil ).

In theory, the ECB of a HP CAN NOT fall below unity ** as
compared to electric strip heat **, IE, it will ALWAYS return at
least
1 KW worth of heat for 1 KW of power to run it ( COP of 1 ).

Well agreed on the last point. However if the alternative source is
gas/oil, the EBP will be reached well before the COP falls close to 1.
Right?

And if the HP is sized so that it can maintain design temp even below
the EBP, then it seems clear that as outdoor temperature drops, further
increase in heat loss simultaneous with reduction in capacity to reach
the TBP, could only result in the cost of electric HP heat exceeding
gas/oil at the TBP.

So I'd say that the statement "below the 'thermal balance point', the
heat pump is STILL putting out economical heat into the house" depends
on

1) alternate heat is resistive strips, such that the EBP cannot ever
realistically be reached - OR -

2) the TBP is above the EBP, and the presence of an EBP implies fossil
fuel alternate heat source; in this case, the HP would never be run
below the EBP because the fossil source would have taken over.

I could see how either 1 or 2 could apply to almost all real world
scenarios, except for gross oversizing.

When I was at my neighbor's diagnosing an (unrelated) electrical
problem, I discovered his HP can maintain 67F indoors with 17F outdoors
with the outdoor unit fan not running (burnt out). His COP can't be
that great in that situation, certainly not as high as 2. It was
installed in 1978, oil backup. He's on his 2nd tank of oil since
installation.

His HP is on a separate meter. This underscores the point that the EBP
is tied to the rates of both electricity and the alternate fuel. That
can change day-to-day, even hour-to-hour because of demand metering.

%mod%